The giant Martian sandstorm of 2018 was not only a wild journey, but also gave us a gas that had not previously been detected in the planet’s atmosphere. For the first time, the ExoMars orbiter took samples of traces of hydrogen chloride, composed of a hydrogen and a chlorine atom.
This gas presents Mars scientists with a new mystery to solve: how it got there.
“We first discovered hydrogen chloride on Mars,” said physicist Kevin Olsen of Oxford University in the UK.
“This is the first detection of a halogen gas in Mars’ atmosphere and represents a new chemical cycle to understand.”
Scientists have been alerting to chlorine-containing gases in Mars’ atmosphere as they could confirm that the planet is volcanically active. However, if hydrogen chloride was produced by volcanic activity, it would only have to increase very regionally and be accompanied by other volcanic gases.
The hydrogen chloride detected by ExoMars did not and did not. It sniffed in both the northern and southern hemispheres of Mars during the dust storm and the absence of other volcanic gases was blatant.
This suggests that the gas was being produced by some other process; fortunately, on Earth we have similar processes that can help us understand what it could be.
It is a multi-step process that requires key ingredients. First, you need sodium chloride (which is normal salt), which is left over from evaporation processes. There are many on Mars, which are believed to be the remains of ancient salt lakes. When a dust storm removes the surface, sodium chloride is released into the atmosphere.
Then there are the Martian polar caps that, when heated during the summer, sublimate. When the resulting water vapor is mixed with the salt, the resulting reaction releases chlorine, which then reacts further to form hydrogen chloride.
Graph showing the potentially new chemistry cycle detected on Mars. (IS IN)
“You need water vapor to release chlorine and you need the byproducts of water, hydrogen, to form hydrogen chloride. Water is key in this chemistry,” Olsen said.
“We also observe a correlation with dust: we see more hydrogen chloride as dust activity increases, a process related to the seasonal warming of the southern hemisphere.”
This model relies on the detection of hydrogen chloride during the next 2019 dust season, which the team is still analyzing.
However, confirmation is still pending. Future and ongoing observations will help to develop a more complete picture of the cycles of the process.
Meanwhile, laboratory experiments, modeling, and simulations will help scientists rule out or confirm the possible mechanisms behind the release of hydrogen chloride into the Martian atmosphere.
The research has been published in Scientific advances.